1. Field of the Invention
The present invention relates to suppressing transient impulses in a power distribution system and more particularly to a device and method for suppressing transient impulses without using an isolation transformer.
2. Description of the Prior Art
Conventional conditioning circuits have been developed to filter and suppress transient impulse conditions from propagating in a power distribution system, for example, electrical noise, severe overvoltage, transient, voltage impulse, current impulse, and the like. The power distribution system can be configured as either a two wire, single phase power distribution system having a power line or a neutral line or as a three wire system having a power line (L), neutral line (N) and a ground line (G). A transient voltage impulse on either the power line or neutral line with respect to the ground line is commonly known as common mode (cm) noise. A transient voltage impulse on the power line with respect to the neutral line is referred to as normal mode (nm) noise. Transient voltage impulses are predominately found on the power line yet can be found on the neutral line, for example, when these are miswired by switching the power and neutral lines. Power conditioning filter circuits operate, for example, to convert a transient voltage to a transient current, and then to inject, shunt or otherwise dump the transient current to the ground line, whereby these currents propagate thereon until reaching absolute ground.
It is important to suppress the transient impulse condition because it can have destructive effects and create problems in electrical equipment such as errors, failures and other abnormalities. Transient voltage and other surge voltage data has been compiled by the IEEE 587 Surge Voltage Working Group and published as ANSI standard C62.41. The worse case surge voltages have been attributed to lightning strikes on the power fine. The worse case surge voltage was noted to have a practical limit of .+-.6000 volts or .+-.6 kilovolts (kv). This limit is governed by the intrinsic capability of electrical wiring systems and devices to withstand this level of surge voltage. The surges were further characterized by their rise time and decay time which is dependent on their position of incidence along the power network. If the incidence occurs on the primary side of a distribution transformer then the incident appears as a one half microsecond (1/2.mu.sec) rise time impulse with a 100 kilohertz (KHz) damped ring wave decay to zero. If the incident is on the secondary side of a distribution transformer then the impulse is unipolar and has been characterized with a 1.0 .mu.sec rise time and 50 .mu.sec decay time to one half (1/2) its peak value. These wave shapes are determined by the incident surge voltage interacting with the impedance of the distribution transformer.
Conventional power conditioning can be classified three ways: (1) simple filters; (2) Transient Voltage Suppressors (TVSS); and (3) isolation transformer based filters and TVSS. Simple filters and TVSS shunt transients to ground. Isolation transformers block transients from the load and ground. The isolation transformer typically is located between the power and neutral lines on the load side and the corresponding power and neutral lines of the line side of the circuit so as to operate as a filter. While placing filters on the power and the neutral lines can suppress transient voltages on these with respect to the ground line, this does nothing to suppress a transient current injected or induced on the ground line from propagating in the ground line of the power distribution system.
A transient current propagating in the ground line can create problems in electrical equipment connected to the power distribution system, even with power conditioning, such as errors, failures and other system abnormalities. In most systems, the ground line has been left unconditioned because of concerns for reliable protection against electrical fault conditions in the electrical equipment drawing power therefrom. I have previously devised a method and system of power conditioning that includes a transformer to aid in suppressing transient currents in the ground line as well as transient voltages on the power and neutral lines, as set forth in my patent application Ser. No. 07/921,337, which is incorporated herein by reference. However, there are many disadvantages of filter circuits configured with transformers such as their bulk, weight and cost.
The present invention overcomes these and other disadvantages to make a device and method of transformerless conditioning of the power distribution system which can match the performance of transformer based filters. The present invention provides a transformerless circuit for filtering common and normal mode noise. The present invention also provides a circuit for conditioning the ground line without using transformers.